Measurement of the blood pressure of a patient historically has been accomplished by means of a noninvasive system whereby the cardiovascular system of the patient does not become invaded in any manner whatever and contamination of the cardiovascular system is not possible. Typically, an inflatable cuff is placed about the patient's upper arm and is inflated sufficiently to close off the underlying brachial artery. Upon slowly releasing pressure from the cuff while watching the pressure gauge or mercury manometer interconnected with the cuff, and listening with a stethoscope placed over the brachial artery distal to the cuff, the pressure at which the Korotkoff sounds appear and disappear are determined and these pressures are the patient's systolic and diastolic pressures, respectively. This method is known as the ausculatory method of indirect or noninvasive measurement of blood pressure. In general, the cuff may be placed around other appendages and inflated to a pressure above systolic pressure and other techniques such as palpation, oscillometry or ultrasound may be used to detect the patient's systolic, diastolic or mean pressure as the cuff pressure is slowly reduced. Intermittent monitoring of blood pressure may be accomplished by means of noninvasive blood pressure measurement systems but in recent times, especially in conjunction with surgical procedures where accurate and continuous pressure monitoring is highly desirable, it has become the practice to achieve continuous monitoring of blood pressure by means of an invasive blood pressure monitoring system wherein an artery of the patient is cannulized and blood pressure is detected and displayed or recorded by means of a blood pressure strain gauge, which is also referred to as a blood pressure transducer. This same technique of direct or invasive blood pressure monitoring may also be used to measure the blood pressure, which cannot be measured by noninvasive methods, at other sites in the cardiovascular system than in the arterial system. For example, the blood pressure in the venous system, the pulmonary vascular system and the chambers of the heart may be measured with this invasive technique. Furthermore, it is a common practice now to use this invasive technique to simultaneously measure several of these pressures in patients during surgery and in intensive care units. In practice, the pressure of the blood of the cardiovasular system of the patient is transmitted to a diaphragm of a strain gauge, through a fluid column, thus causing yielding of the diaphragm in accordance with the blood pressure of the patient. Yielding of the diaphragm is detected by means of an electrical system that develops an electrical signal that is directly proportional to the amount of strain that results from deflection of the transducer diaphragm. This electrical signal is amplified and otherwise conditioned and transmitted to the input of an oscilloscope and is displayed as a waveform on the screen of the oscilloscope. This amplified and conditioned electrical signal may also be transmitted to the input of various types of recorders or other display devices, such an analog meters or digital displays. Throughout the rest of this application, the oscilloscope will be cited as a typical display or recording device and the amplifier and signal conditioners will be considered to be part of the oscilloscope.
In order for the oscilloscope to properly display the waveform signal being emitted from the transducer, the transducer electrical signal must be zeroed with respect to a base line representing zero or atmospheric pressure on the oscilloscope. To accomplish zeroing of the blood pressure transducer electrical signal, atmospheric pressure, or some other zero reference pressure, must be communicated to the diaphragm of the transducer. The diaphragm, being at the zero reference pressure, will emit an electrical signal representing the zero reference pressure and this pressure signal will appear on the oscilloscope in visual form. The oscilloscope is then adjusted to bring the zero reference pressure signal into registry with a base line representing zero or atmospheric pressure on the screen of the oscilloscope. After this has been done, the blood pressure monitoring system is then considered to be zeroed.
After zeroing of the blood pressure monitoring system, it is then necessary to check the calibration of the blood pressure monitoring system, thus ensuring that the transducer and oscilloscope are capable of proper electrical signals within the pressure range that is expected. The typical method for calibration of the blood pressure monitoring system is to interconnect the transducer either directly or through a system of valves and tubing with the output hose of a mercury manometer and to pump the manometer up to the maximum expected pressure above the zero reference pressure, for example, 250 millimeters of mercury. With the blood pressure transducer thus connected and a pressure of 250 millimeters of mercury above the zero reference pressure applied to the diaphragm of the transducer, the transducer will be emitting an electrical signal representing a pressure of 250 millimeters of mercury above the zero reference pressure. This signal is transmitted to the oscilloscope screen for visual display and, if the blood pressure transducer and oscilloscope are properly calibrated, will provide a visual reading of 250 millimeters of mercury. Thus, the blood pressure transducer is subjected to a known pressure above the zero reference pressure, i.e. 250 millimeters of mercury, and this known pressure is visually inspected on the oscilloscope screen. If the pressure on the oscilloscope screen reads other than 250 millimeters of mercury, then the blood pressure monitoring system must be adjusted to obtain a reading of 250 millimeters of mercury. If the blood pressure monitoring system is of the type which cannot be adjusted or if it is not possible to adjust it to obtain a reading of 250 millimeters of mercury, then the blood pressure monitoring system is defective and must not be used to monitor patients' pressure until the defect is isolated and corrected. The defect may be in the transducer or oscilloscope or both.
Although zeroing and calibrating of invasive blood pressure monitoring systems is quite simple, there are serious problems associated with typical methods of accomplishing zeroing and calibrating. The fluid column connecting the cardiovascular system of the patient to the transducer diaphragm should remain closed and sterile. However, opening the blood pressure monitoring system to atmospheric pressure for zeroing allows bacteria and other contaminants to enter the fluid column that is interconnected with the cardiovascular system of the patient. When this occurs, it is possible for bacteria to migrate through the fluid system to the cardiovascular system of the patient and cause bacterial infection thereof and thus endanger the health of the patient. It is also possible in normal use that this fluid which has been contaminated will be infused into the patient, endangering the health of patient. Further, when mercury manometers, such as are typically mounted on the walls of hospital facilities, are interconnected with the blood pressure monitoring system for calibration, the manometer is not in sterile condition. It is possible, therefore, for bacteria and other contaminants to be injected into the fluid of the blood pressure monitoring system, and these bacteria may migrate to the cardiovascular system of the patient or the contaminated fluid may, in normal use, be infused into the cardiovascular system of the patient. It is desirable, therefore, to provide a method and apparatus for invasive blood pressure monitoring wherein the system is maintained in a closed condition at all times with respect to the bacteria-containing external environment, and thus, the possibility of introducing bacteria and other contaminants into the cardiovascular system of the patient is avoided.
When an invasive blood pressure monitoring system is interconnected with the cardiovascular system of a patient and a transducer zeroing and calibrating operation is in progress, a valve in the tubing leading from the cannula to the transducer must be closed. During zeroing, if the valve or stopcock is left open, the arterial pressure of the patient will pump the patient's blood through the tubing and out of the orifice which has been opened to atmospheric pressure for zeroing. The results will be loss of blood which in the most severe case could lead to exsanguination of the patient. During calibration of the blood pressure monitoring system, the diaphragm within the transducer dome will be subjected to a pressure which typically exceeds the blood pressure of the patient. If the above-referenced stopcock in the tubing to the patient is open during attempted calibration, air will be injected through the tubing to the patient and air embolus will result.
In order to avoid the previous hazards of contamination and air embolus which may result during the calibration of an invasive blood pressure monitoring system, the system is sometimes simply zeroed but not calibrated. The result of this omission can be very serious. For example, if the pressure monitoring system is not properly calibrated, the physician may believe the patient's blood pressure is high when in reality it is low. The physician would then, in all probability, inject a medicament to lower the patient's blood pressure. Inappropriate administration of such a medicament could cause the patient to go into shock or cardiac arrest. Conversely, if the physician is falsely led to believe the patient's blood pressure is low when in reality it is high, the physician would then, in all probability, inject a medicament to raise the patient's blood pressure. Inappropriate administration of such a medicament could cause the patient to experience a stroke. Other possible harmful consequences of inaccurate invasive blood pressure measurements include inappropriate infusion of fluid causing heart failure, performing unnecessary surgery or failing to perform necessary surgery. This possibility of causing a physician to inappropriately prescribe for or treat a patient thus leading to a worsening of the patient's condition is considered to be the most serious problem associated with invasive blood pressure monitoring.
At the time of the initiation of use of an invasive blood pressure monitoring system, it is necessary that the site of the fluid-air interface at which the system is opened to atmosphere to establish the zero reference pressure be leveled with respect to the right atrium of the patient's heart, which is typically the mid-thoracic level when the patient is supine. This leveling ensures that the transducer senses only blood pressure and not a hydrostatic pressure caused by a difference in height between the fluid-air interface and the patient's heart, as well. Because there is no convenient, accurate process available by which to ensure that the fluidair interface is in proper alignment with the patient's heart, often the leveling is accomplished by simple line of sight over a distance of several feet. This process can be dangerous to the patient by causing inaccurate pressure measurement, especially when the blood pressure measured is expected to be quite low. For example, the normal range for pulmonary wedge pressure to 6 to 12 millimeters of mercury, and each centimeter of leveling error will result in the addition to or subtraction from the blood pressure measurement of a hydrostatic pressure equal to 3/4 millimeter of mercury. Therefore, a leveling error of only four centimeters will cause a measurement error of 25%-50 % of the actual pressure. It is desirable, therefore, to provide a convenient means for positively ensuring accurate leveling of the fluid-air interface with respect to the mid-thoracic line of the patient and thus ensure elimination of any pressure measurement error due to misalignment of the fluid-air interface with the patient's heart.
Blood pressure transducers are typically of quite expensive nature, in the order of $400 to $500, and being of delicate nature, are quite easily damaged due to movement of personnel about the patient during a surgical procedure or in an intensive care unit. Many manufacturers of blood pressure transducers provide quality holders for the transducers themselves and thus provide the transducers with adequate protection, but, provision is not made in these holders for other devices that are commonly employed in conjunction with blood pressure transducers. For example, constant flush systems are often employed in conjunction with blood pressure transducers to ensure against coagulation of the patient's blood at the cannula insertion site, which might otherwise result in blockage of the blood pressure monitoring system and thus inaccurate blood pressure measurement. Typical continuous flush systems are interconnected with the blood pressure transducers and extend therefrom in an unprotected position where they are often struck and broken by personnel attending the patient. It is possible, also, that application of inadvertent force to a continuous flush device or the plumbing interconnected therewith will result in damage to the blood pressure transducer. Moreover, breakage of the continuous flush device or its plumbing can result in contamination of the cardiovascular system of the patient. With rapid worldwide acceptance of invasive cardiovascular hemodynamic monitoring, there is a need for a special holder to hold one or more continuous flush systems and transducers to prevent damage thereto; level the fluid-air interface with the mid-thoracic line of the patient; and house a system that provides a closed sterile path with bacteria filtering in order to prevent contamination while zeroing and calibrating the pressure monitoring system.
In view of the foregoing, it is a primary feature of the present invention to provide a novel method and apparatus for accomplishing zeroing and calibrating of blood pressure monitoring systems without in any way endangering the patient from the standpoint of bacterial contaimination, loss of blood or inappropriate administration of medicaments or other inappropriate treatment based on inaccurate pressure measurements due to reluctance to calibrate the pressure monitoring system for fear of contamination thereof.
It is also a feature of this invention to provide a novel system for invasive monitoring of the blood pressure of patients wherein positive alignment of the fluid-air interface used to establish the zero reference pressure with the mid-thoracic line of the patient may be achieved simply and efficiently, thereby preventing erroneous blood pressure readings that might otherwise occur due to improper leveling.
It is an even further feature of this invention to provide a novel apparatus for holding and protecting one or more pressure transducers of like or different designs and the continuous flush systems and other plumbing attached thereto to prevent breakage of the transducers, continuous flush systems or plumbing which might result in costly loss of equipment, inaccurate pressure measurement or contamination of the cardiovascular system of the patient.
It is also a feature of this invention to provide a novel system for zeroing and calibrating of a plurality of blood pressure monitoring systems, each consisting of a blood pressure transducer and oscilloscope trace or other display device, used simultaneously for one patient, wherein a single zeroing and calibrating mechanism may be utilized for selectively zeroing and calibrating each blood pressure monitoring system, while maintaining each of the systems in an environmentally closed condition during the zeroing and calibrating procedure. Furthermore, all such blood pressure monitoring systems interconnected with one zeroing and calibrating mechanism may be zeroed and calibrated simultaneously thereby assuring identical, superimposed trace deflections on the oscilloscope in response to the calibrating pressure.
It is also a feature of this invention that if a plurality of pressure transducers of different designs are supported and protected in one holder and interconnected with a single zeroing and calibrating mechanism, all transducer types need not be held with the sensing diaphragm at the same height in order for the pressure monitoring system to indicate accurate pressure measurements without errors introduced by hydrostatic pressure differences.
Other and further objects, advantages and features of the invention will become obvious to one skilled in the art upon an understanding of the illustrative embodiment about to be described and various advantages, not referred to herein, will occur to one skilled in the art upon employment of the invention in practice.